Positron emission tomography (PET) is a medical imaging technique that provides in vivo functional information on physiological processes. In PET, positron-emitting radionuclides on biologically active molecules, such as fluorodeoxyglucose (FDG), are introduced into a patient. A radionuclide in the patient's body decays and emits a positron, which undergoes an annihilation with a nearby electron, subsequently generating a pair of 511 keV gamma ray photons that travel in nearly opposite directions. If a pair of gamma ray photons are detected within a coincidence timing window by two detectors in an array of PET detectors, a coincidence event is recorded for the line of response (LOR). The LOR is a line between the two detectors that have detected the photon pair. Each event is sorted in an array called a sinogram.
PET image reconstruction is used to reconstruct the three-dimensional distribution of the radiotracer in the patient's body from the measured sinogram data. The spatial distribution of the radiotracer is called an activity or emission image, or simply an image, and an estimate of the unknown true image provided by an image reconstruction process is called a reconstructed image.